The present invention relates to a pin for connecting carbon electrodes, and a process for preparing the inventive pin. More particularly, the invention concerns a pin for connecting carbon electrodes, such as graphite electrodes, having a thread design and shape which can alleviate points of high mechanical stress concentration on the pin and electrode.
Carbon electrodes, especially graphite electrodes, are used in the steel industry to melt the metals and other ingredients used to form steel in electrothermal furnaces. The heat needed to melt metals is generated by passing current through a plurality of electrodes, usually three, and forming an arc between the electrodes and the metal. Currents in excess of 100,000 amperes are often used. The resulting high temperature melts the metals and other ingredients. Generally, the electrodes used in steel furnaces each consist of electrode columns, that is, a series of individual electrodes joined to form a single column. In this way, as electrodes are depleted during the thermal process, replacement electrodes can be joined to the column to maintain the length of the column extending into the furnace.
Generally, electrodes are joined into columns via a pin (sometimes referred to as a nipple) that functions to join the ends of adjoining electrodes. Typically, the pin takes the form of opposed male threaded sections, with at least one end of the electrodes comprising female threaded sections capable of mating with the male threaded section of the pin. Thus, when each of the opposing male threaded sections of a pin are threaded into female threaded sections in the ends of two electrodes, those electrodes become joined into an electrode column. Commonly, the joined ends of the adjoining electrodes, and the pin therebetween, are referred to in the art as a joint.
Given the extreme mechanical, electrical and thermal stresses that the joint (and indeed the electrode column as a whole) undergoes, the joint must be able to withstand such conditions and survive continuous consumption of the electrode column until it reaches the electric arc tip to be as efficient and economic as is desired. The joint must also be capable of quick and easy assembly. Due to the harshness of the environment in which the joint is expected to operate, these joints often break or degrade sufficiently (through observable cracks and/or splits) that significant amounts of graphite material are lost. As is apparent, this increases electrode consumption and decreases efficiency. It is believed that much of this degradation is caused by stresses inherent to conventional joint design, and especially points of high or concentrated stress, and the differences in material properties between the joined elements.
What is desired, therefore, is a threaded pin for connecting carbon electrodes, the pin having a shape and thread design and placement capable of alleviating or mitigating the points of high stress concentration that can cause joint failure. Especially desirable is such a pin that incorporates features that can distribute the stresses more evenly about the joint and increase the strength capability of the threaded pin.
It is an object of the present invention to provide a threaded pin for connecting carbon electrodes, the threaded pin incorporating features that can alleviate points of high stress concentration that can cause joint failure.
It is another object of the present invention to provide a threaded joint which is shaped so as to reduce high stress concentrations.
It is still another object of the present invention to provide a threaded pin having threads shaped so as to alleviate high stress concentrations.
These objects and others that will become apparent to the artisan upon review of the following description can be accomplished by providing a threaded pin for connecting carbon electrodes, where the threaded pin comprises a body having a central axis running along its length, two end portions, a midpoint lying between the two end portions and threads extending from the body, the threads each having at least two side walls. The side wall closest to the midpoint of the body of at least a plurality of the threads has an angle of between about 75xc2x0 and about 90xc2x0, and more preferably at least about 85xc2x0, with respect to the central axis of the body. Indeed, a majority of the threads, and most preferably substantially all of the threads have the side wall closest to the midpoint of the body having an angle of between about 75xc2x0 and about 90xc2x0, and more preferably at least about 85xc2x0, with respect to the central axis of the body.
The inventive threaded pin advantageously has a body that is generally circular in cross-section and has a channel extending therethrough from one of the end portions to the other. The channel should have a diameter of about 15% to about 55% of the diameter of the body of the pin at its midpoint, and a diameter of about 35% to about 70% of the diameter of the body of the pin at its two end portions.
Preferably, at least a plurality of the threads of the inventive pin have truncated ends. More specifically, at least a plurality of the threads located at or near at least one of the end portions of the body of the pin have truncated ends. Moreover, a line extending along the tops of the untruncated threads will advantageously have an angle of at least about 12xc2x0 with respect to a line parallel to the central axis of the pin.